This invention relates to a power supply for supplying welding energy for a percussion welding operation and, more particularly, to a capacitor-discharge type of power supply for this purpose.
Percussion welding is an arc welding process in which: (a) the contact surfaces of two work pieces are heated and melted by an electric arc drawn between them, and (b) during or immediately following such arcing, the work pieces are rapidly driven together by a welding force to form a weld between the molten surfaces. A good general description of percussion welding can be found in the Metals Handbook, 8th Edition, Volume 6, pages 177-186, published in 1971 by the Amercian Society for Metals, Metals Park, Ohio. In one form of percussion welding, one of the work pieces is provided with a nib or small projection that is brought into engagement with the other work piece at the start of the welding operation. At a desired instant, a relatively high welding current is caused to flow between the two work pieces via the projection, rapidly heating the projection and causing it to disintegrate, thus forming an arc between the two work pieces in the region where the projection was located. This arc quickly melts the immediately-adjacent work-piece areas so that the work pieces are bonded together when these molten areas engage after being driven together by the welding force. The arc is extinguished when the work pieces reach engagement following arcing. The welding energy, as the term is used hereinafter, is that energy developed by current flowing through the arc prior to the work pieces' reaching engagement. This energy is, of course, directly dependent upon current, arc voltage, and the arcing time.
Some percussion welding power supplies derive their welding energy directly from a conventional a.c. source through a welding transformer without relying upon energy storage. Problems present with such a power supply are: (1) its operation imposes a sudden, large power drain on the a.c. source, especially when the supply line between the source and welding equipment is long, (2) the voltage available from the secondary of the welding transformer is severely limited and, as a result, is susceptible to being driven too rapidly to zero by the arc voltage developed upon arc-initiation, especially when welding refractory metals, which have characteristically high arc voltages, (3) this rapid decrease in voltage available from the secondary makes it difficult to develop the required high welding force in time to be applied while the work piece surfaces are still at the ideal temperatures for welding, and (4) the rapid decrease in secondary voltage makes it very difficult to develop a good weld over a large area, especially when welding refractory metals.